Syntax: JavaMaxHeapSize=number of bytes
Default value: 67108864 (64mb)
Specifies the maximum--not initial--size the Java heap can reach. The Java Virtual Machine (JVM) starts out at 16MB of heap space and most of it is uncommitted. If the JVM needs more heap than it currently has, it will expand the heap in increments but will not exceed the maximum. Exceptions such as "java.lang.OutOfMemoryError" indicate that a heap has reached its maximum size. You can specify the number of bytes directly or use the suffix "MB" to indicate megabytes, for example, specifying "64MB" is the same as specifying "67108864."
Resource obtained from : http://www.lntoolbox.com/en/notesini-reference/bycategory/serverconfiguration/14-Server_Configuration/1148-javamaxheapsize.html
Hope this helps :)